Friday, December 14, 2012

Stellar Disruption from Accumulated Tidal Heating

Sagittarius A* is a 4 million solar mass supermassive black hole located in the centre of the galaxy. A disk of young stars surrounds Sagittarius A* and within the inner radius of this disk is a group of stars known as the S-stars. The S-stars are the closest stars known to orbit the supermassive black hole and they are young main sequence stars that are each several times more massive than the Sun. With new instruments, more stars with lower masses are expected to be found orbiting even closer to Sagittarius A*.

Figure: An infrared image of the
galactic centre.

Stars orbiting sufficiently close to
Sagittarius A* can accumulate tidal heating from multiple close approaches with
the supermassive black hole. The amount of accumulated tidal heating can eventually
disrupt the star even though its closest approach is a few times the tidal
disruption distance which is the distance within which the gravity of the
supermassive black hole becomes sufficiently strong to unbind the star. With
each close approach, the star experiences tidal heating which causes the
deposition and accumulation of heat in the star’s interior. As a result of this
added source of energy, the star expands and becomes more bloated which reduces
the binding energy of the star. Furthermore, tidal heating becomes stronger as
the size of the star increases. This results in a runaway process and after a
sufficient number of close approaches; the star becomes unbound, leading to its
disruption.

Massive stars are more susceptible to
eventual disruption by accumulate tidal heating. This is because the tidal
disruption distance is larger for a massive star than for a low mass star. As a
result, a low mass star can get closer to Sagittarius A* before being subjected
to tidal disruption. Such a mechanism may explain the lack of high mass stars
existing very close to Sagittarius A*. In light of new instruments that can
detect lower mass stars, it can be conceived that there could be an increase in
the fraction of low mass stars closer to Sagittarius A* since low mass stars
can approach closer to the supermassive black hole without being disrupted by accumulate
tidal heating.